1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an engine.
2. Description of the Related Art
If the ratio of the total amount of air fed to the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel and the reducing agent fed to the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position, is referred to as an air-fuel ratio of the exhaust gas flowing through the certain position, it is well known that if an engine, in which a lean air-fuel mixture is burned, has a NO.sub.X absorbent arranged in the exhaust passage thereof, the NO.sub.X absorbent absorbs NO.sub.X therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releases the absorbed NO.sub.X therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower. In the engine, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent is made temporarily rich to thereby release the absorbed NO.sub.X from the NO.sub.X absorbent and to reduce the released NO.sub.X by unburned hydrocarbon HC and carbon monoxide CO in the exhaust gas.
However, fuel and the lubrication oil contain sulphur containing components and, thus, the exhaust gas also contains sulphur containing components. The sulphur containing components in the form of SO.sub.X, for example, are absorbed in the NO.sub.X absorbent in the form of SO.sub.4.sup.2-, together with NO.sub.X. However, the sulphur containing components are not released from the NO.sub.X absorbent even when the air-fuel ratio of the inflowing exhaust gas is merely made rich. Thus, the amount of the sulphur containing components absorbed in the NO.sub.X absorbent increases gradually. However, if the amount of the sulphur containing components in the NO.sub.X absorbent increases, the NO.sub.X absorbing capacity of the NO.sub.X absorbent gradually becomes smaller and, at the last, the NO.sub.X absorbent can hardly absorb NO.sub.X therein.
However, the NO.sub.X absorbent releases the absorbed sulphur containing components therefrom in the form of SO.sub.2, for example, when the oxygen concentration in the inflowing exhaust gas becomes lower with the temperature of the NO.sub.X absorbent being high. Thus, Japanese Unexamined Patent Publication No. 8-61052 discloses an exhaust gas purifying device for an engine with a plurality of cylinders, in which the cylinders are divided to a pair of cylinder groups. In the device, a lean air-fuel mixture is burned in one cylinder group to form the exhaust gas including the rich oxygen, a rich air-fuel mixture is burned in the other cylinder group to form the exhaust gas including the rich unburned HC and CO, and the exhaust gases are introduced to the NO.sub.X absorbent so that the unburned HC and CO are burned in the NO.sub.X absorbent to increase the temperature of the NO.sub.X absorbent. At this time, the air-fuel ratio of the entire exhaust gas is made rich and, thus the sulphur containing components are released from the NO.sub.X absorbent. Accordingly, the NO.sub.X absorbent is reactivated.
On the other hand, in general, the exhaust gas purifying catalyst arranged in the exhaust passage of the engine is not activated for a while after the engine has been started and, therefore, good exhaust gas purification of the exhaust gas purifying catalyst cannot be expected at this period. Therefore, it is well known that a catalyst for the exhaust gas purification at the start of the engine, such as a three-way catalyst, is additionally arranged in the exhaust passage upstream of the exhaust gas purifying catalyst. If such an additional catalyst is referred to as a start catalyst, the start catalyst is arranged adjacent to the engine and the heat capacity thereof is made smaller. Therefore, the start catalyst becomes active more rapid than the exhaust gas purifying catalyst after the engine is started, and reduces the amount of unburned HC and CO discharged to the outside air before the exhaust gas purifying catalyst becomes active.
However, the NO.sub.X absorbent cannot be reactivated sufficiently if the start catalyst is applied to the exhaust gas purifying device disclosed in the '052 publication. Namely, in this case, the exhaust gases of the cylinder groups are mixed and flow through the start catalyst and, then, flow into the NO.sub.X absorbent. In other words, the exhaust gas including the rich oxygen and the exhaust gas including the rich unburned HC and CO flow into the start catalyst generally simultaneously. As a result, most of the unburned HC and CO are burned or consumed in the start catalyst and, therefore, the amount of the unburned HC and CO fed to the NO.sub.X catalyst is reduced. Accordingly, the NO.sub.X absorbent is not heated sufficiently and, therefore, the sulphur containing components are not released from the NO.sub.X absorbent sufficiently.